1. Field of the Invention
The present invention relates to an optical wavelength shifter, and more particularly to an optical wavelength shifter for shifting the wavelength of light by varying timings at which the light and a pump pulse are incident on a nonlinear refractive medium
2. Description of the Prior Art
Intense, ultrafast, and broad-spectral-width pulses close to white-light called continuum or supercontinuum are generated by passing picosecond laser pulses through a nonlinear refractive medium having a nonlinear refractive index.
For spectral broadening, it has been known to utilize a phase modulation wherein the spectral range of the laser is broadened when the laser pulse passes through a nonlinear refractive medium. As the laser pulse passes through the medium, it causes a refractive index change. This in turn induces a phase change which causes a frequency sweep within the pulse envelope. This is called self phase modulation (SPM). Induce phase modulation (IPM) and cross phase modulation (XPM) are also known in the art. In IPM, a phase modulation is performed using an intense laser pulse serving as pump pulse. More specifically, when a weak pulse of a different frequency passes through the medium whose refractive index was changed by the intense laser pulse, the phase of the weak optical field can be modulated by the time variation of the index of refraction originating from the primary intense pulse.
Similarly, in XPM, light is phase shifted when a pump pulse and the light to be modulated are simultaneously incident on the nonlinear refractive medium, wherein the phase shift depends on the peak power of the pump pulse. A differential of the phase shift corresponds to a frequency modulation, hence the phase modulation is equivalent to the frequency modulation. Consequently, the spectrum of light can be broadened or an optical wavelength can be changed through the phase modulation by way of XPM. As disclosed in a paper entitled "Induced-frequency shift of copropagating ultrafast optical pulses" by P. L. Baldeck et al, announced in "American Institute of Phys" published Jun. 6, 1988, it has been known that the central wavelength is shifted caused by the change of a timing at which the pump pulse and the light to be modulated are incident on the nonlinear refractive medium. Since the phase modulation depends on the peak power of the pump pulse in XPM, there is a problem that a large peak power pump pulse is required.